a) Field of the Invention
The present invention relates to a hydrodynamic bearing device in which a substantially conically-shaped shaft bush and a bearing sleeve are relatively elevated by dynamic pressure of lubricant fluid so that their rotations are supported in a non-contact manner, and to a recording disk drive equipped with it.
Also, the present invention relates to a hydrodynamic bearing device having a hydrodynamic bearing member that supports a rotary shaft with dynamic pressure of lubricant fluid, and to a disk driving device.
b) Description of the Related Art
In recent years, hydrodynamic bearing devices in which various bodies-to-be-rotated can be supported at a high speed rotation in a stable manner have been developed. In such bearing devices a conical hydrodynamic bearing device, wherein a shaft bush having a substantially conical inclined dynamic pressure surface is relatively rotatably inserted into a shaft sleeve having a conical inclined dynamic pressure surface, and the lubricant fluid such as oil is filled in a substantially conical inclined bearing space which is created in the gap between the inclined dynamic pressure surface of the bearing sleeve and the inclined dynamic pressure surface of the shaft bush.
Then, a dynamic pressure generating means composed of properly-shaped recessed grooves is cut on at least one of the inclined dynamic pressure surfaces of the shaft bush and bearing sleeve. When the shaft bush and the bearing sleeve are relatively rotated, the lubricant fluid is pressurized by the dynamic pressure generating means to generate dynamic pressure. Using the dynamic pressure of the lubricant fluid, the shaft bush and the bearing sleeve are relatively elevated in both the radial and thrust directions so that both members are rotatably supported in a non-contact manner. For example, Patent References 1 through 4 are known:                Patent Reference 1: JP H7-7886 Publication        Patent Reference 2: JP H10-339318 Publication        Patent Reference 3: JP 2002-174226 Publication        Patent Reference 4: JP 2003-97547        
As mentioned above, in a conical hydrodynamic bearing device, the dynamic pressure surfaces formed on the bearing sleeve and shaft bush are substantially conically inclined with respect to the rotary shaft; in order to obtain a stable amount of relative float between the fixed member and the rotary member, it is desirable to make the open angles of the inclined dynamic pressure surfaces (i.e. the angle corresponding to the vertex angle of the conically-shaped inclined hydrodynamic pressure surface) as great as possible so that the dynamic pressure in the thrust direction is increased.
In recent years, as a bearing device for rotating bodies-to-be-rotated at a high speed and with high precision among various rotation drive devices, a hydrodynamic bearing device in which dynamic pressure is generated in lubricant fluid to support a rotary shaft in a non-contact manner has been developed. In such a hydrodynamic bearing device, the present inventors have proposed a hydrodynamic bearing device in which a thrust hydrodynamic bearing portion, SB, is configured as in FIG. 23, in order to manufacture a thin device on the whole. In other words, in the thrust hydrodynamic bearing portion SB shown in the figure, a rotary member (rotary hub) 303 is joined to a rotary shaft 302 that is rotatably supported by a hydrodynamic bearing portion (bearing sleeve) 301; the end surface of the rotary member 303 on the axially inner side (the bottom surface in FIG. 23) in the center area and the end surface of the hydrodynamic bearing member 301 on the axially outer side (the top end surface in FIG. 23) are closely opposed to each other and the above-mentioned thrust hydrodynamic bearing portion SB is formed in a portion of the area created between the opposing members in the thrust direction.
Inside of the bearing space at the thrust hydrodynamic bearing portion SB, suitable lubricant fluid (not illustrated) is injected; as a dynamic pressure generating means for the lubricant fluid, spiral-shaped dynamic pressure generating grooves are cut along the circumference. By the pressurizing function of the dynamic pressure generating grooves, dynamic pressure is generated in the lubricant fluid to obtain a desirable elevating force in the thrust direction.
Also, two radial hydrodynamic bearing portions RB, RB are provided in the axial direction in the area between the inner circumferential wall surface of the hydrodynamic bearing member 301 and the outer circumferential wall surface of the rotary shaft, which are opposed to each other in the radial direction. Inside of the bearing space of each radial hydrodynamic bearing portion RB, the lubricant fluid (not illustrated) is injected continuously from the above-mentioned thrust hydrodynamic bearing portion SB. As a dynamic pressure generating means for the lubricant fluid, herringbone-shaped dynamic pressure generating grooves are cut along the circumference. By the pressurizing function of the dynamic pressure generating grooves, dynamic pressure is generated in the lubricant fluid to obtain a desirable elevating force in the radial direction.
In the above-mentioned hydrodynamic bearing device, the bearing space is continuous from the two radial hydrodynamic bearing portions RB, RB to the thrust hydrodynamic bearing portion SB, and the lubricant fluid is filled inside the continuous bearing space without interruption.
Problems to be Solved
However, if the open angle of the inclined dynamic pressure surface is set to be large (to support the body-to-be-rotated at a high speed in a stable manner), the entire device becomes large in the radial direction. Even if one attempts to make the device small using this configuration, this configuration cannot be adopted due to the constraints by the magnetic drive portions. Also, if the open angle of the inclined dynamic pressure surface becomes too large, the dynamic pressure in the radial direction becomes low, possibly degrading bearing rigidity.
Further, as the open angle of the inclined dynamic pressure surface is set to be large, the rotational centrifugal force is increased at the outer circumference of the inclined bearing space. Accordingly, the action force by the rotational centrifugal force may become larger than the fluid retaining force of the fluid sealing portion. Thus, even when a fluid sealing portion is provided, the lubricant fluid may leak outside.
Also, in the conical hydrodynamic bearing device, since the inclined bearing space is created between both inclined dynamic pressure surfaces on the bearing sleeve and bearing bush, it is advantageous in that dynamic pressure can be obtained in the radial direction and in the thrust direction simultaneously; on the other hand, however, the pressure balance of the lubricant fluid filled in the inclined bearing space is easily affected by a minute dimensional error in each member, and the differential pressure between both ends of the inclined bearing space becomes great. For this reason, it is difficult to obtain the amount of relative float between the fixed member and rotary member easily and with certainty, and thus it is difficult to obtain a suitable bearing rigidity.
In the hydrodynamic bearing device having such a configuration, the gap dimension of the radial hydrodynamic bearing portion RB and the shape of the dynamic pressure generating groove (the groove length) can be affected by a machining error during manufacturing, bringing an unbalanced shape in the axial direction. Because of this, the different pumping action forces P1, P2 as shown by the lengths of the arrows in FIG. 24 are obtained (P1>P2) in the radial hydrodynamic bearing portion RB at the bottom of the figure, for example. This unbalanced condition in the radial hydrodynamic bearing portion RB cannot be cancelled completely even if the hydrodynamic bearing portions or the neighbors thereof are able to communicate with each other by a circulating hole such as pressure-adjusting bypasses. Especially, if the pressure of the lubricant fluid inside the fluid reservoir 304 provided in the center portion of the bearing space that includes the above-mentioned radial hydrodynamic bearing portion RB and thrust hydrodynamic bearing portions SB becomes a negative pressure, which is smaller than the atmospheric pressure, bubbles may be generated in the lubricant fluid. If such bubbles invade the inside of each hydrodynamic bearing portion, desirable dynamic pressure cannot be obtained and the bearing property is greatly degraded.